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Kate Becker: Hunting high and low for gravitational waves

By Kate Becker

For the Camera

Posted:
10/12/2017 04:04:17 PM MDT

Barry C. Barish, left, and Kip S. Thorne attend a press conference at California Institute of Technology after receiving the 2017 Nobel Prize in Physics on Oct. 3. They shared the award with Rainer Weiss, professor emeritus of physics at MIT. Thorne and Barish helped co-found, along with Weiss, the Laser Interferometer Gravitational-wave Observatory, which made the first-ever direct detection of gravitational waves, or ripples in the fabric of space and time, which had been predicted by Albert Einstein 100 years earlier. (David McNew / Getty Images)

Kate Becker The Visible Universe

This year's physics Nobel Prize announcement came with a sort of joyful inevitability.

Few were surprised that Rainer Weiss, Barry Barish and Kip Thorne, three physicists whose ideas were key to the first direct detection of gravitational waves, each got a share of the 2017 prize. The three worked on the gravitational wave detector LIGO, a flagship experiment of the National Science Foundation, involving hundreds of scientists and decades of planning, construction and patient waiting. Using a system of lasers and mirrors, LIGO directly measured vibrations in space-time radiating out from the collision of two black holes more than a billion light years from Earth.

The discovery was the billion-dollar ribbon tying up the long project of verifying general relativity. General relativity reimagines gravity as the warping of space-time by massive objects. So, when those massive objects accelerate, gravitational waves ripple through space-time like water waves through the ocean. If this logic isn't obvious to you, don't worry: it wasn't obvious to Einstein, either. For 20 years after first proposing gravitational waves as part of general relativity, he waffled over whether they were real or not.

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Physicists have been accumulating indirect evidence for gravitational waves since 1974, when astrophysicists Russell Hulse and Joseph Taylor found the first binary pulsar. A binary pulsar is a dead star orbiting another dead star, thumping out radio waves as it whirls around. By clocking the pulses coming from the pulsar, Hulse and Taylor could figure out the particulars of the binary system, including the strange fact that it seemed to be losing energy over time. The big insight: The missing energy was just what general relativity predicted should be leaking away in the form of gravitational waves. Hulse and Taylor won the 1993 Nobel Prize for their discovery.

Yet the results were still circumstantial. When LIGO powered up in 2002, scientists hoped it could do better by measuring gravitational wave vibrations directly. And for eight years, it sat, listening, waiting, without detecting a single gravitational wave. In 2010, it was shut down for a major upgrade. Then, on Sept. 14, 2015, the freshly-restarted LIGO felt a tiny wobble. The signal was clearly discernible, such a tidy fit with theorists' predictions that many LIGO researchers assumed it was actually a sham slipped into the data just for practice. But it was not a drill: It was the very first direct detection of gravitational waves. Since then, there have been three more similar detections.

It's tempting to think of this as the end of the story — but it's actually just the beginning, and the dawn a totally new way of exploring the universe. Until now, everything we knew about the universe came to us by way of light: The kind we can see with our eyes, and the kind we can only see with telescopes. Accessing gravitational waves is like gaining a new sense.

But just as there are many different kinds of light waves, there is also a whole spectrum of gravitational waves. LIGO and similar Earth-bound detectors only have access to the high-frequency end of that spectrum, but gravitational waves rumbling out from the most massive black holes are hiding at the bottom of the spectrum. Researchers are already searching for those low-frequency waves using a scrappy approach that can be done on a shoestring budget, using only what's already available: that is, pulsars and radio telescopes. Strung out across the galaxy, pulsars are like cosmic signaling stations. By comparing when pulses arrive at telescopes across the Earth, researchers can search for the signature of passing gravitational waves.

Now the irony part: The Arecibo Observatory and the Green Bank Telescope, mainstays of the pulsar timing telescope network, are being squeezed out of the National Science Foundation's budget. Losing either one would mean giving up half the sensitivity of the North American gravitational wave search experiment, NANOGrav, says West Virginia University astrophysicist Maura McLaughlin, who chairs the collaboration. It will be years before next-generation radio telescopes can join the search.

LIGO is a dazzling feat of technology and brainwork, and complementary experiments like NANOGrav can help it shine — if they don't get lost in the glare.

Rainer Weiss, professor emeritus of physics at MIT, looks at a prototype he built in 1974 of a radio frequency modulated test inteferomater following a press conference after it was announced Oct. 3 he shares a Nobel Prize in Physics for LIGO. Half of the prize was awarded to Weiss with the other half split by Kip S. Thorne and Barry C. Barish of the California Institute of Technology, in recognition "for decisive contributions to the LIGO detector and the observation of gravitational waves." (Scott Eisen / Getty Images)

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